Inflammation
is the first response of the immune system to infection or
irritation and may be referred to as the innate cascade. Inflammation
is characterized by the following quintet: redness (rubor), heat (calor),
swelling (tumor), pain (dolor) and dysfunction of the organs involved
(functio laesa). The first four characteristics have been known since ancient
times; functio laesa was added to the definition of inflammation by Rudolf Virchow in 1858.

Characteristics

The
redness and heat are caused by the increased blood supply to the affected area.
Blood
vessels are vasodilated upstream of an infection while capillary permeability
to the affected tissue is increased, resulting in a loss of blood plasma.
Vasoconstriction downstream of the infection further increases edema or swelling. The
swelling distends the tissues, compresses nerve endings, and thus causes pain.
The white blood cells or
leucocytes take on an important role in inflammation; they extravasate from the
capillaries
into tissue, and carry on as phagocytes picking up bacteria and
cellular debris. They may
also aid by walling off an infection and preventing its spread.

If
inflammation of the affected site persists, released cytokines IL-1 and TNF will
activate endothelial cells to upregulate receptors VCAM, ICAM-1, E-selectin, and
L-selectin for various immune cells. Receptor upregulation increases extravasation
of neutrophils, monocytes, activated T-helper and T-cytotoxic, and memory T and
B cells to the infected site.

Leukocytes
and cytokines involved in inflammation

Various
leukocytes are involved in the initiation and maintainance of inflammation. These
cells can be further stimulated to maintain inflammation through the action of
adaptive cascade through lymphocytes: T cells, B cells, and antibodies. These
inflammation cells are:

Mast
cells which release histamine and prostaglandin in response to activation of stretch
receptors. This is especially important in cases of trauma.

Macrophages
which release TNF-±, IL-1 in response to activation of toll-like receptors.

Resolution

When
inflammation subsides, the damaged tissue is repaired. Depending on the severity
of the inflammation and the type of tissue involved repairs may or may not be
complete; in minor inflammations of the skin, for example, the tissue is
capable of complete regeneration whereas in nervous
tissue regeneration may be more limited and the damaged cells may be replaced
with scar tissue.